The study of post-coital fruit fly poop started in 2011 with this paper. The authors, Paola Cognigni, Andrew Bailey, and Irene Miguel-Aliaga, wanted to use the fly to study signals between the gut and the nervous system. Studying the feedback between gut and brain signals in vertebrates–let alone humans–is a daunting task. There are way too many variables, neurons, everything. Flies are pretty complex organisms in their own right but, you know, less so than we are. Cognigni and colleagues discovered a lot digging around in fruit fly guts. They found that the intestine regulates how much the fly eats as well as the fluid and ion balance in the gut.

Some of the gut’s functions involve the brain. But we’re here to talk about sex, not brains. It turns out that sex actually alters the physiology of the female fruit fly’s gut. And it’s not just sex in general, but specifically a single protein within the semen: sex peptide. Sex peptide actually does require the nervous system to do its dirty work in the mated female fly, but for its function in the gut, it uses the enteric nervous system rather than the brain. Receipt of sex peptide does a lot to a girl fly: she stops wanting to mate with other males, lays lots and lots of eggs, sleeps less, and eats more. But when it comes to poop, sex peptide does something really weird: it slows down the digestive process, making the poo much more compact. Plus, there’s much more of it.

Cognigni, et al. devised a system for measuring the amount and quality of female fly poop. They noted that mated females, in contrast to virgins, produced more of what the authors call “reproductive oblong deposits” or RODs (which are, of course, rod-shaped). By using females that don’t make mature eggs, they were even able to determine that ROD production was independent of another sex peptide effect: pumping out massive numbers of eggs. Instead, RODs seem to be the by-product of a water-conserving strategy used by mated females. The authors backed up this claim by showing that they could induce ROD production in virgin females, and even sometimes males, by reducing the water in their food. Sex peptide literally changes the way the intestine processes food.

“I’m gonna have to hit the can after this”

So, in 2011 we knew that 1) sex, and specifically sex peptide, makes females poo more, that 2) the poo is much more concentrated, and 3) the effect is independent of egg production. But this still left a few questions.

Sex peptide, or SP, as I mentioned, affects many post-mating processes in fruit flies. Some of SP’s effects only last for a day or so, for example a jump in yolk-protein production and egg development rate. But most of SP’s famous effects are long-term, lasting for up to 2 weeks. This includes the suppression of mating. And while we learned a lot about the role of SP in fly fecal consistency from Cognigni, et al., they only tracked the poo of mated females for about 72 hours.

But what happens after that?!

Fear not, we have the answer. Soon after the 2011 paper was published, a bright young, bushy-tailed graduate student in the Wolfner lab at Cornell University (that’s where I used to be!) decided to dig deeper into the squalid world of fruit fly fecality. The student, Jennifer Apger-McGlaughon, set up the fly poop measurement system in the lab and got to work. The burning question: is the change in fecal output, caused by sex peptide, a short-term or long-term effect?

A poop plate from a mated female. The flies are fed blue food, so that the “blue-ness” of the poo can be measured. RODs are very dark blue. Round poo drops are varying shades of blue. Image by J. Apger-McGlaughon.

First, she confirmed what Cognigni had showed in the previous report: the proportion of RODs in the female’s post-coital output is significantly increased compared to virgins. But she took it a step further and showed that the effect lasts for at least 7 days–and doesn’t diminish at all during that time. Furthermore, she nailed down the fact that the RODs are caused by sex peptide by mating females to males that lack the protein. Those females’ poo looked just like a virgin’s. No water-saving for them.

The authors also showed that sex peptide loses most of its effect on the digestive tract if it can’t be normally released from sperm tails, where it binds after mating. Sex peptide doesn’t work alone: several other proteins in Drosophila semen are needed to glue this tiny protein onto sperm tails. Later, unknown proteins cut sex peptide off of the sperm, so that it can continually signal to the female that she has, in fact, already mated. When Apger-McGlaughon used males that had a mutated form of sex peptide–one that could bind sperm but not be cut off of it–ROD production decreased by nearly half on the second day after mating, and was mostly wiped out by the third day. A similar effect was seen when one of the other proteins that helps stick sex peptide to sperm was deleted from the male.

Based on the experiments with mutated sex peptide or a deleted semen protein, sex peptide’s effect on fly poop consistency can be broken down into short- and long-term effects.

Another interesting result was that, in contrast to the 2011 paper, Apger-McGlaughon actually found that egg production wasn’t completely independent of the sex peptide effect on ROD production. Females without a germline (ie: that don’t make any eggs) still produced RODs after mating, but whereas normal mated females’ poo was over 20% RODs between 1 and 2 days after mating, “eggless” females only produced about 5% RODs during this timeframe. The apparent inconsistency might be explained by the different types of “eggless” females used in the two studies. The Cognigni paper used females that have a germline, but whose eggs arrest in development. The females used in the current paper don’t make eggs at all. Sex peptide still affects the time it takes for food to digest in the completely eggless females, but to a much lesser extent than in females that make eggs, even if those eggs never reach maturity.

Based on all this, plus other papers that have studied food intake after mating, we can start to build a cohesive picture about what is going in with lady flies’ eating and pooping habits after they have sex. Here is my scientific summary:

But how does this affect my life??

Yeah, it doesn’t. Not right now anyway. That’s the great thing about basic science: it doesn’t have to. The point of basic research is to learn as much about the world as possible, so that the information can be put to good use when it’s needed. We don’t need to know how it’s going to be useful. We just have to do good science.

I guess if you need a concrete reason why you should care about this work, other than the fact that IT’S ABOUT FLY POOP, well…think about this: with every detail that’s added to the sex peptide pathway, we build a more complete picture of a neural network that extends to every behavior of the fly–the decision to mate or not, the amount the female fly sleeps, eats, excretes, how much energy she spends on egg production, how sperm get used to fertilize eggs…and on and on. We’re nowhere close to building this detailed a picture of a neural process like this in mammals. Scientists have mapped the specific neurons in the female that respond to sex peptide, and we’re starting to understand how those neurons control all these different aspects of fly behavior.

If you can’t imagine how that information might one day be useful for some applied purpose, well, you just don’t have much of an imagination.